The present invention relates to a circuit for controlling the maximum current in a MOS power transistor used for driving a load connected to earth.
FIG. 1 of the appended drawings shows a typical circuit of the prior art used to drive a load connected to earth GND. This circuit includes a MOS power transistor M1, for example, an n-channel transistor, with its drain connected to a direct-current supply voltage V.sub.S and its source connected to the load. If the load L is inductive, a free-wheeling diode D1 is arranged in parallel therewith in order, each time the transistor M1 is cut off, to dissipate the energy which the load itself has stored during the previous stage in which M1 was conductive.
The transistor M1 is used as a switch and therefore operates in the triode region. In this region, the transistor M1 operates with a low voltage V.sub.DS and behaves essentially as a voltage-controlled resistance, the resistance value of which is lower, the greater the quantity by which the gate-source voltage (V.sub.GS) exceeds V.sub.DS +V.sub.TH, where V.sub.TH is the threshold voltage of the MOS transistor, that is, a voltage such that the transistor is cut off when V.sub.GS is less than V.sub.TH.
In the circuit shown in FIG. 1, it is of fundamental importance that V.sub.DS should be small since the lower this voltage is, the greater is the power supplied to the load L.
The gate of the transistor M1 is driven by a charge pump circuit CP of known type. This circuit enables the gate capacitance of the transistor M1 to be charged to a voltage greater than the supply voltage V.sub.S, although with currents of low intensity.
If the source of M1 is accidentally short-circuited to earth GND or if the resistance of the load L is very small, the transistor M1 no longer operates in the triode region, but operates in a region in which its drain current depends on V.sub.GS and is almost independent of V.sub.DS.
To limit the power dissipated by the power transistor M1 under these circumstances (in order to prevent damage), it is necessary to limit the drain current by reducing the gate voltage.
The problems connected with the control of the maximum current in a circuit configuration of the type shown in FIG. 1 relate essentially to the frequency stability of the entire system and to the precision of the control.
FIGS. 2 and 3 of the appended drawings show two different known solutions for controlling the maximum current which can flow through the MOS power transistor M1.
The solution shown in FIG. 2 uses an amplifier A which may be a normal operational amplifier or a transconductance operational amplifier. The non-inverting input (+) of this amplifier is connected to the drain of M1 and its inverting input (-) is connected to the negative terminal of a reference-voltage generator V.sub.R, the positive terminal of which is connected to the positive terminal of the supply voltage V.sub.S. The amplifier A also has a supply terminal s which is connected to V.sub.S. The output of the amplifier is connected to the gate of M1.
A resistor R.sub.S is connected between the drain of M1 and the supply voltage V.sub.S.
The current supplied to the gate of M1 by the charge pump CP and the current flowing in the load L are indicated I.sub.CP and I.sub.L, respectively.
FIG. 3 shows another solution according to the prior art. In FIG. 3 the same alphabetical references have again been attributed to parts and elements already described above.
In the solution of FIG. 3, the resistor R.sub.S is between the source of M1 and the load L. The reference-voltage generator V.sub.R is between the load and the non-inverting input (+) of the operational amplifier A. The inverting input of the amplifier is connected to the source of M1.
With both the solutions described with reference to FIGS. 2 and 3, the maximum current which the MOS transistor M1 can supply to the load L is: EQU I.sub.Lmax =V.sub.R /R.sub.S
The current-regulator circuits of FIGS. 2 and 3 use operational amplifiers and, in order to operate in a stable manner, must be suitably compensated. The gate capacitance of M1, which, in both cases, is connected to the output of the operational amplifier A, involves the introduction of an additional pole in the frequency response which, with the use of an operational amplifier which itself has to be compensated, may make the system unstable, particularly when the MOS transistor used has large physical dimensions and hence a high gate capacitance.
If the amplifier A of the diagrams of FIGS. 2 and 3 is a transconductance amplifier, it must have a very high gain g.sub.m in order to enable precise regulation of the maximum current in the transistor M1 since, in practice, the current I.sub.CP coming from the charge pump CP is never known sufficiently accurately.
If the gate capacitance of the MOS transistor M1 is used to compensate the transconductance operational amplifier, the high gain g.sub.m may involve problems of frequency instability, particularly if the MOS transistor used has small dimensions and hence a small gate capacitance.
The object of the present invention is to provide a circuit for controlling the maximum current in a MOS power transistor used for driving a load connected to earth, which enables the current to be controlled more precisely with a low-gain transconductance amplifier which has very good frequency stability characteristics and simple circuitry.
According to the invention, this object is achieved by a control circuit of the type specified above, the main characteristic of which lies in the fact that the charge pump circuit is connected to the supply terminal of the transconductance operational amplifier.